This invention relates to position sensing and, more particularly but not exclusively, to a position sensor based on magnetic principles and employing elements of soft magnetic material. Such material is conveniently a high permeability material with an axis of easy magnetisation. Suitable materials for this purpose include the materials conventionally used in electronic article surveillance (EAS) techniques; these materials are well known in the art and do not require further description herein.
In previous patents, in particular U.S. Pat. Nos. 6,144,300, 6,373,388, 6,323,770, 6,323,769, 6,329,916, and 6,371,379, we have described and claimed novel techniques for spatial magnetic interrogation and novel tags. The technology described in U.S. Pat. Nos. 6,144,300, 6,373,388, 6,323,770, 6,323,769, 6,329,916, and 6,371,379 is based on exploiting the behavior of magnetic materials as they pass through a region of space containing a magnetic null. In particular, these earlier applications describe, inter alia, how passive tags containing one or more magnetic elements can perform as remotely-readable data carriers, the number and spatial arrangement of the elements representing information.
In the above applications we described a number of possible system embodiments employing either permanent magnets or electromagnets to create the magnetic null. We also described several system implementations some of which are particularly appropriate for tags employing very low coercivity, high permeability magnetic elements. These implementations work by detecting harmonics of a superimposed low-amplitude alternating interrogation field.
In a later patent, U.S. Pat. Nos. 6,230,379, we describe arrangements which work by detecting the baseband signals generated by the passage of the tag through the magnetic null, without the need for any superimposed alternating interrogation field. Specific designs for readers are described in PCT/GB97/02772. The content of these above-mentioned patent applications (hereinafter termed xe2x80x9cthe prior FN applicationsxe2x80x9d) is incorporated herein by reference thereto.
The present invention preferably utilises techniques and materials such as described in the prior FN applications, or components of such techniques and materials, in order to provide a position sensor and method.
Preferably, two or more elements of soft magnetic material are utilised in the invention and the relative positions of the magnetic elements are changed by the movement which is to be sensed. The positions of the magnetic elements are sensed in a remote way by a suitable means. One suitable means is the spatial magnetic interrogation technique described in WO 96/31790.
One embodiment of the technique disclosed in WO 96/31790 may be defined as a method of determining the position of a magnetic element, or the relative positions of two or more magnetic elements on a tag, the magnetic element(s) having non-linear magnetic properties and a preferential axis of magnetisation, which is characterised by the steps of: (1) applying a magnetic field to an interrogation zone where the magnetic element(s) is or are located, or is or are expected to be located, said magnetic field being: (i) generated by magnetic field generating means positioned independently of said magnetic element(s); (ii) such that a magnetic null as defined in WO 96/31790 is generated within said interrogation zone, the magnetic null being contiguous with regions where the applied magnetic field is sufficient to saturate the, or a part of the, magnetic element(s); and (iii) such that the direction in which the resolved component of the magnetic field is zero is or can be aligned with the preferential axis of magnetization of the or each magnetic element; (2) causing relative movement between said magnetic field and said magnetic element in the direction of the preferential axis of magnetisation of the magnetic element(s) such that at least a portion of the magnetic element, or of each of the magnetic elements in turn, becomes magnetically saturated and then enters the magnetic null; (3) detecting the magnetic response of the or each magnetic element during said relative movement; and (4) determining the position of the magnetic element, or the relative positions of the magnetic elements, from the time(s) of occurrence of the or each magnetic response.
The present invention is particularly appropriate for medical applications, where the status of movable elements or describes implanted inside the human body need to be ascertained externally.
Position sensors based an many different technologies have been developed. In general they utilise the movement of one element with respect to a reference in order to create a readily-measured effect. For example, one type of optical position sensor employs a glass strip carrying a pattern of opaque lines. An optical source on one side of the strip is detected by a photodiode on the other side whenever there is no opaque region between them. Linear motion of the strip relative to the optical sensor pair results in electrical pulses being generated by the photodiode, and position relative to a datum may be determined by counting the number of pulses generated. This arrangement, in common with most sensors providing electrical output, requires detectable features (in this case the optical pattern) to be close to the sensing means (in this case the light beam). This generally makes such sensors inconvenient for implanting inside the human body because of their bulk, and/or because of the need for connecting wires.
According to a one aspect of the present invention, there is provided a method of sensing the position of a first element with respect to a second element, the first element being movable with respect to the second, which comprises:
(a) fixing a first magnetic marker to said first element;
(b) fixing a second magnetic marker to said second element;
(c) sensing the positions of the two markers by a remote magnetic sensing technique.
The invention is based on detecting small elements of soft magnetic material. In the simplest arrangement, one element is attached to a moving or movable member, and another to a reference member. Using a remote magnetic sensing technique, the positions of the two magnetic elements can be detected. The preferred magnetic sensing principle is that described in WO 96/31790. This employs a null in a magnetic field to locate the position of a piece of soft magnetic material. By scanning the null across the plane containing the two magnetic elements, their presence can be detected, and by relating the time of occurrence of the detections to the scanning pattern and speed, their separation can be deduced.
Thus, in one embodiment of the invention, the method includes the steps of: (1) applying a magnetic field to said magnetic markers, said magnetic field being: (i) generated by magnetic field generating means positioned independently of said magnetic markers; (ii) such that a magnetic null as defined in WO 96/31790 is generated, the magnetic null being contiguous with regions where the applied magnetic field is sufficient to saturate the, or a part of the, magnetic markers; and (iii) such that the direction in which the resolved component of the magnetic field is zero is or can be aligned with the preferential axis of magnetisation of the or each magnetic marker; (2) causing relative movement between said magnetic field and said magnetic markers in the direction of the preferential axis of magnetisation of the magnetic markers such that at least a portion of each of the magnetic markers in turn becomes magnetically saturated and then enters the magnetic null; (3) detecting the magnetic response of each magnetic marker during said relative movement; and (4) determining the relative positions of the magnetic markers from the times of occurrence of the or each magnetic response.
A variant on this arrangement uses fixed magnetic elements at the extremes of travel of the moving element, i.e. 3 elements in total. An advantage of this scheme is that the separation of the two fixed elements is known, so that the position of the moving element as a fraction of the fixed element spacing can be determined directly from the ratio of the spacings between the 3 detection signals, without the need for knowledge of the null scanning speed. This eliminates the need for accurate calibration of the reader equipment, and means that, for some applications, a very simple manually scanned sensor is practical.
The same principle can be employed in a rotary position sensor, this time using a rotating magnetic nullxe2x80x94for example, such as described in our patent application PCT/GB97/03389 (WO 98/26312, published Jun. 18, 1998). In this case the fixed element is placed at a reference angle. The position of the moving or movable element is determined by detecting the point between successive detections of the reference element at which the moving element is detected. Since there is the potential for a 180xc2x0 ambiguity in this system, for some applications it is necessary to ensure that the fixed marker responds differently to interrogation with the gradient field in different directions along its axis. This may be done by applying a local magnetic bias to the second magnetic marker, e.g. from a further magnetised element; one technique for this is described in GB9717475.9 and PCT/GB98/02479. Alternatively, the bias may be achieved by suitably shaping the element, as described in application GB9800064.9 (Sentec Limited, assigned to Flying Null Limited) and PCT/GB99/00017.
Advantageously, the remote magnetic sensing technique is that described in WO 96/31790. The magnetic markers are preferably soft magnetic materials of high permeability and having an axis of easy magnetisation. The soft magnetic axis will generally be aligned with the direction of motion when that motion is linear. It will also generally be aligned with the direction of scanning the magnetic null when the techniques of WO 96/31790 are used.